In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. The multiple optical signals are multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate channels. Each channel may be at an associated wavelength that is separated from adjacent channels by a defined channel-spacing, e.g. according to a channel plan established by the International Telecommunications Union (ITU). The entire range of wavelengths that may be transmitted on the system is known as the system bandwidth. Systems may utilize their system bandwidth to carry a desired number of channels with desired modulation format and bit rate.
To satisfy increasing demand for transmission capacity in optical transmission systems, spectral efficiency has been increased using a number of techniques. Multi-level modulation techniques and coherent receivers have been used, for example, to allow increased transmission rates and decreased channel spacing, thereby increasing the aggregate spectral efficiency (SE) of a WDM system. In a multi-level modulation format multiple data bits are encoded on a single transmitted symbol. For example, in quadrature amplitude modulation (QAM) formats, multiple bits-per-symbol may be represented using phase shift keying (PSK) with or without amplitude shift keying. In a quadrature phase-shift keying (QPSK) format, which may be referred to as 4QAM, four phases may be used without amplitude shift keying to represent two bits-per-symbol. An 8QAM format uses phase shift keying and amplitude shift keying to represent three bits-per-symbol and a 16QAM modulation format uses phase shift keying and amplitude shift keying to represent four bits-per-symbol. The spectral efficiency of a modulation format may be defined by number of bits-per-symbol (bits/symbol) encoded by the modulation format. An 8QAM format has a higher spectral efficiency (3 bits/symbol) than a QPSK format (2 bits/symbol).
While use of multi-level modulation formats may increase the overall spectral efficiency and transmission capacity of a system, formats having a higher spectral efficiency may exhibit a lower Q-factor at the same optical signal-to-noise ratio (OSNR) compared to formats having a lower spectral efficiency. For example, in an exemplary system including one-hundred ten spans of 60 km using ultra p-type fiber, if the repeater output power is limited to −4.1 dB per channel, a signal modulated using 8QAM may exhibit a Q-factor of about 4 dB lower than a signal modulated using QPSK. In such a system, achievable Q-factors for the 8QAM signal may not be sufficient to reliably meet system requirements when manufacturing imperfections, system aging and other performance impairments are taken into account. To achieve higher Q-factor for the 8QAM signal, higher OSNR may be required.
Operating with high OSNR may require high optical channel output power and high amplifier pump power, especially for wide system bandwidths. Delivering high power levels can present a significant technical and economic challenge, in particular, in undersea systems where the electrical power for the entire cable must be transported along the cable. In this scenario, the ability to realize increased performance may be impeded by a limited amount of available power.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.